Intense interest has focused on the NMDA subtype of glutamate receptor because of its roles in development, synaptic transmission, memory, and pathological situations such as ischemia and epilepsy. Although NMDA receptors are modulated by many endogenous substances, the control of receptor function by physiological concentrations of protons is particularly important, not only because the interstitial pH is highly dynamic, but also because of the extracellular acidification that accompanies ischemia and seizures. These perturbations in pH could serve as negative feedback to inhibit NMDA receptor function, and thereby limit seizure duration or glutamate-mediated damage during ischemia. My preliminary studies have shown that the 5th alternative exon of the NR1 subunit renders receptors proton-insensitive, and this effect can be mimicked by endogenous polyamines. This finding provides an opportunity to approach proton control of NMDA receptor function in structural terms, and additionally raises the possibility that the different splice variants of the NMDA receptor might be related to the selective vulnerability of central structures to ischemia-induced damage or seizure initiation. The long-term objective of this project is to determine at the structural and functional level how the 5th NR1 exon controls receptors in normal and pathological situations. The five main goals are: (1) To identify the structural determinants within the NR1 5th exon that control proton inhibition. (2) To determine at the biophysical level how the NR1 5th exon controls the receptor's proton sensitivity. (3) To identify compounds that modulate the NMDA receptor's proton sensitivity, and determine whether these compounds exert their effects in a manner similar to the 5th exon. (4) To investigate the link between the proton sensitivity of recombinant NMDA receptor splice variants and their cytotoxic potential. (5) To determine whether the 5th exon is differentially expressed in the central nervous system. These experiments will help to explain at the structural, anatomical, and functional levels how the 5th exon influences receptor function. In addition, these experiments will help to define an important regulatory site on the NMDA receptor, and identify compounds that act at that site.